The present invention relates to a device for moving solid particles, especially one designed as cooling mixer, having a substantially horizontal jacketed tank, which is passed along its longitudinal axis by a shaft connected to a drive motor and carrying radially extending tools, which when rotating act to mix solid particles contained in the bulk material by setting them into forced movement, and having further charging and discharge openings arranged on the tank and at least two partition walls, provided each with at least one passage opening and arranged in the tank between the charging and the discharge openings, transversely to the longitudinal axis.
A device of this kind has been known from DE-AS 11 12 968.
Devices for mixing solid particles using rotating tools, comprising a horizontal cylindrical tank and a shaft with mixing tools arranged coaxially in the tank, have been known before in the form of batch-type and continuous-type mixers.
In batch-type operation, the machine, which is used as mixer, drier, reactor, or cooler, is at first charged with the bulk material or materials and/or additives to be processed. The charging process, once completed, is followed by a processing operation by which the bulk materials are mixed, cooled, dried, heated, disintegrated or agglomerated. This process may be accompanied also by reactions which may give rise to new products, or may release gases which are then evacuated through suitable vapor pipes. Upon completion of the treating process, the products are discharged from the machine by a separate discharging operation.
For the purpose of determining the time required for preparing solid-particle mixtures by means of batch-type mixers, it is necessary to sum up the charging time required for the bulk materials to be mixed, the discharging time for the finished mixture, and the mixing time as such is required by the batch-type mixer to mix the bulk materials with themselves and with each other by means of the rotating mixing tools. This results in longer mixing times for the batch-type mixer, as compared with continuous-type mixers. In continuous operation, the waiting times required for charging and discharging the mixer do not occur. In continuous operation, the bulk materials to be processed are supplied to the mixer continuously, and at the same time the finished mixture is discharged continuously. This is possible also in the case of bulk materials which are to be subjected to thermal treatment or to be cooled in continuous operation. If a cooling process is to be carried out in continuous operation, the hot solid-particle material is introduced into the cooling mixer, while at the same time the cooled bulk material is extracted from the other end of the cooling mixer.
In continuous operation, however, certain difficulties are encountered with respect to controlling the transport behavior of the bulk materials to be cooled in such a way as to prevent, for example, cooled bulk material to get mixed up with bulk material of higher temperature or even hot bulk material. If reverse mixing occurs, then the mixing temperature of the bulk material in the product space will always be higher than the desired end temperature. And in addition, reverse mixing causes cooled particle populations to stay in the product space longer than the permissible time. Such particle populations may be destroyed by the mixing tools, being subjected to prolonged mechanical treatment. This makes it necessary, when cooling a flow of bulk materials in continuous operation, to keep the rate of reverse mixing between treated and untreated materials as low as possible. However, it is often not possible with the known devices to control the cooling process in this way since rigidly predetermined process parameters often prevent corresponding operating modes and since, on the other hand, the time required for reducing the temperature in bulk materials by heat transfer to a cooling agent cannot be reduced at desire.
Known mixers normally operate at a rigidly preset rotational speed, specific to the particular device and adapted to the particular process. In order to maintain the cinematic similarity, the dimensionless value Fr (Froude number) is introduced, instead of the rotating speed n. Fr is a dimension independent of the drum diameter, describing the ratio of centrifugal acceleration to gravitational acceleration. The Froude number can be described by the following formula: ##EQU1## wherein m is the mass of particles, r is the radius of the drum, g is the gravitational acceleration, defined by the formula g=r.multidot.n.sub.c.sup.2 .multidot.4.pi..sup.2, .omega. is the angular velocity of the mixing tool, and n.sub.c is the critical speed.
In the case of the continuous-type solid particle mixer known from the doctoral thesis of 1984 entitled "Untersuchungen zur Schuttgutbewegung beim kontinuierlichen Feststoffmischen" (Studies on the movement of bulk material during continuous mixing of solid matter) (Process Engineering Faculty of the Stuttgart University), the mean dwelling time of the bulk material in the solid particle mixer can be controlled via the bulk mass stored in the product space and via the bulk mass flow. The product space of this solid-particle mixer extends from the delimiting surface at the mixer end to a weir provided in the cylindrical drum, directly in front of the product discharge pipe. The bulk mass stored between the weir and the product charging pipe is subjected to heavy reverse mixing at rotational speeds of Fr.gtoreq.4, as illustrated for example by the diagrams on page 108 of this paper, where the axial dispersion coefficient D is plotted as a function of the Froude number Fr. However, as has been stated before, this is a disadvantage when a bulk material flow is to be cooled continuously.
As can be seen in FIG. 2 of the drawing, the mixer known from DE-AS 11 112 968 comprises a jacket, which is not explained in detail in the specification. However, if effective thermal treatment is to be ensured, it is not sufficient merely to provide a tank jacket; the latter then necessarily has to be designed in such a way as to permit effective cooling or heating of the product flowing through the tank. In addition, an uncontrolled flow of product is permitted through the openings at the bottom of the partition walls.